Recent developments in wireless communications and the emergence of high data rate services have consumed almost all the accessible spectrum making it a very scarce radio resource. Spectrum from very low frequencies to several GHz range has been either dedicated to a particular service or licensed to its providers. It is very difficult to find sufficient bandwidth for new technologies and services within accessible spectrum range. Contrarily, studies in different parts of the world reveal that the licensed and/or dedicated spectrum is underutilized leaving unused bands at different frequencies. These idle bands; however, cannot be used by non-licensed users due to current spectrum management practices throughout the world. This fact forced the regulatory authorities and academia to rethink the spectrum allocation policies. This resulted in the idea of spectrum sharing systems, generally known as cognitive radio, in which non-licensed or secondary users can access the spectrum licensed to the primary users. Many techniques and procedures have been suggested in the recent years for smooth and transparent spectrum sharing among the primary and secondary users. The most common approach suggests that the secondary users should perform spectrum sensing to identify the unused bands and exploit them for their own transmission. However, as soon as the primary user becomes active in that band, secondary transmission should be switched off or moved to some other idle band. A major problem faced by the secondary users is that the average width of the idle bands available at different frequencies is not large enough to support high data rate wireless applications and services. A possible solution is to integrate few idle bands together to generate a larger bandwidth. This technique is also known as spectrum aggregation. Generally, it is proposed to build the transmitter with multiple radio frequency chains which are activated according to the availability of idle bands. A combiner or aggregator is then used to transmit the signal through the antenna. Similarly, a receive antenna can be realized through multiple receive RF chains through a separator or splitter. Another option is to use orthogonal frequency division multiplexing in which sub-carriers can be switched on and off based on unused and active primary bands, respectively. These solutions are developed and analyzed for direct point to point links between the nodes. In this work, we analyze spectrum aggregation for indirect links through multiple relays. We propose a simple mechanism for idle band integration in a secondary cooperative network. Few relays in the system partly facilitate the source to aggregate available idle bands and collectively all the involved relays provide an aggregated larger bandwidth for the source to destination link. We analyze two commonly used forwarding schemes at the relays; namely, amplify-and-forward and decode-and-forward. We focus on outage probability of the scheme and derive a generalized closed form expression applicable to both scenarios. We analyze the system performance under different influential factors and reveal some important trade-offs.


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